Measure and prepare all ingredients. Preheat the oven to 350°F (175°C).
In a large bowl, mix together the egg, peppermint extract, vegetable oil, and water.
Whisk the mixture together using a hand whisk until well combined.
Add the brownie mix to the wet ingredients.
Mix everything with the hand whisk until the batter is smooth and there are no lumps.
Prepare a baking pan by lightly spraying the bottom with cooking oil.
Line the baking pan with parchment paper.
Transfer the brownie mixture into the prepared baking pan.
Smooth the batter with a silicone spatula.
Bake the brownies according to the instructions on the box, which is typically 25-30 minutes.
After removing the brownies from the oven, immediately sprinkle peppermint chips on top and lightly press them once with a silicone spatula. Be careful not to press too many times to avoid melting the chocolate chips and turning them into a peppermint spread.
Allow the brownies to cool in the pan for a minimum of 30 minutes before cutting.
Once cooled, remove the brownies from the baking pan.